What is the actual problem?
Mostly that this isn't documented anywhere that I could find.
The normal practice is for CSS to be in stylesheets (internal or external) vs inline. It took me forever to figure out that in this case, it had to be inline. Being in a stylesheet didn't matter. I don't think that is intuitive so it would be useful for you to document it. Better still, IMHO, would be for it to not matter where height and width are set if that is possible.
I feel it is good to document anything needed to make something work, including something like viewport-fit. That way, if something isn't working for someone they can more easily figure it out.
BTW,
your figuring out how to get around limitations like the lack of real fullscreen on the iphone is hugely appreciated.